The present invention is directed to hydrocarbon combustion in general. In particular, the invention provides a method for the combustion of fuel rich hydrocarbons in diesel engines or oil burners, wherein formation of soot is substantially suppressed in the fuel combustion chamber by suppressing soot forming reactions, which otherwise proceed during combustion of fuel being rich in higher hydrocarbons.
Data from the literature on ion chemistry in flame reactions, e.g. in H. F. Calcote, D. G. Keil, xe2x80x9cThe role of ions in soot formationxe2x80x9d, Pure and Appl. Chem., Vol. 62, No. 5 (1990), show that formation of soot proceeds by an ionic mechanism, wherein propargyl or cyclo propenyl chemi-ions C3H3+ is the main precursor of soot.
C3H3+ ions are formed during combustion of fuel by reaction of methine radicals CH* with oxygen:
2CH*+02⇄2CHO++2exe2x88x92xe2x80x83xe2x80x83(1)
followed by the reactions:
CHO++CH2⇄CH3++COxe2x80x83xe2x80x83(2)
CH3++C2H2⇄C3H3++H2xe2x80x83xe2x80x83(3)
The highly reactive propargyl ions react with hydrocarbons being present in fuel combustion gas. Soot is finally formed in a fast sequence of condensation reactions initiated by addition of propargyl ions to acetylene molecules in the gas and further hydrocarbon additional reactions with the initial ionic condensation products of propargyl ions and acetylene molecules.
Soot in exhaust gas from fuel combustion represents a serious environmental and health risk and removal or reduction of soot emission in exhaust gas is at present prescribed by environmental regulations.
We have found that application of an electrostatic field in a fuel combustion chamber during combustion of fuel rich hydrocarbons reduces or substantially prevents formation of soot in the combustion chamber of e.g. a diesel engine and oil burner. Formation of C3H3+ and higher ionic hydrocarbon species resulting from additional reactions is substantially suppressed by the electrostatic field presumably because of unfavourable equilibrium conditions for the formation of the CHO+ precursor in the above reaction (1) in an electrostatic field.
An electrostatic field inside a fuel combustion chamber is obtained by arranging one or more electrodes in the chamber. Voltage may be applied to the electrodes constantly or at variable levels depending on fuel load. In a diesel engine, the electrodes may be a part of the fuel preheating system installed in the engine combustion chamber.
Use of an electrostatic field in the depression of soot formation is known from U.S. Pat. No. 4,091,779. The problem with this known method is that soot deposits on surface of the electrode during operation. Soot on the electrode surface decreases the strength of the electrostatic field in the combustion chamber.
We have thus found that when coating the conductive part of the electrode with a porous ceramic material being permeable to oxygen, soot is continuously burnt off from the electrode surface.
Preferably, at least the part of the electrode extending into the combustion chamber is coated or otherwise provided with zirconium dioxide.
Based on the above findings and observations, this invention provides a method for the combustion of fuel being rich in hydrocarbons at reduced formation of soot by applying within a fuel combustion chamber an electrostatic field and thereby suppressing formation of soot from ionic species being formed during the combustion of the fuel.
The method of this invention may further be applied to reduction of other noxious compounds in exhaust gas from hydrocarbon combustion including reduction of nitrogen oxides, dioxines and aldehydes.